diff --git a/src/uproot/writing/_cascadetree.py b/src/uproot/writing/_cascadetree.py
index 9ecd2e87f..bf5491cec 100644
--- a/src/uproot/writing/_cascadetree.py
+++ b/src/uproot/writing/_cascadetree.py
@@ -663,11 +663,20 @@ def extend(self, file, sink, data):
 
             if datum["counter"] is None:
                 if datum["dtype"] == ">U0":
-                    lengths = numpy.asarray(awkward.num(branch_array.layout))
+                    awkward = uproot.extras.awkward()
+
+                    layout = awkward.to_layout(branch_array)
+                    if isinstance(
+                        layout,
+                        (awkward.contents.ListArray, awkward.contents.RegularArray),
+                    ):
+                        layout = layout.to_ListOffsetArray64()
+
+                    lengths = numpy.asarray(awkward.num(layout))
                     which_big = lengths >= 255
 
                     lengths_extension_offsets = numpy.empty(
-                        len(branch_array.layout) + 1, numpy.int64
+                        len(layout) + 1, numpy.int64
                     )
                     lengths_extension_offsets[0] = 0
                     numpy.cumsum(which_big * 4, out=lengths_extension_offsets[1:])
@@ -685,7 +694,7 @@ def extend(self, file, sink, data):
                         [
                             lengths.reshape(-1, 1).astype("u1"),
                             lengths_extension,
-                            awkward.without_parameters(branch_array.layout),
+                            awkward.without_parameters(layout),
                         ],
                         axis=1,
                     )
@@ -693,8 +702,8 @@ def extend(self, file, sink, data):
                     big_endian = numpy.asarray(awkward.flatten(leafc_data_awkward))
                     big_endian_offsets = (
                         lengths_extension_offsets
-                        + numpy.asarray(branch_array.layout.offsets)
-                        + numpy.arange(len(branch_array.layout.offsets))
+                        + numpy.asarray(layout.offsets)
+                        + numpy.arange(len(layout.offsets))
                     ).astype(">i4", copy=True)
                     tofill.append(
                         (
diff --git a/tests/test_1264_write_NumPy_array_of_strings.py b/tests/test_1264_write_NumPy_array_of_strings.py
new file mode 100644
index 000000000..0872bf417
--- /dev/null
+++ b/tests/test_1264_write_NumPy_array_of_strings.py
@@ -0,0 +1,18 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/uproot5/blob/main/LICENSE
+
+import pytest
+import uproot
+import os
+import numpy as np
+
+
+def test(tmp_path):
+    newfile = os.path.join(tmp_path, "example.root")
+
+    with uproot.recreate(newfile) as f:
+        f["t"] = {"x": np.array(["A", "B"]), "y": np.array([1, 2])}
+        f["t"].extend({"x": np.array(["A", "B"]), "y": np.array([1, 2])})
+
+    with uproot.open(newfile) as f:
+        assert f["t"]["x"].array().tolist() == ["A", "B", "A", "B"]
+        assert f["t"]["y"].array().tolist() == [1, 2, 1, 2]